Newest line in operation.

[Manchester77]

Does the DLR to Stratford International count. If so then that'll be pretty high (2011)

 

[Eagle]

Quite a lot of that is the old North London line though (only the curvy bit between Stratford and Stratford International is completely new).

 

[Ploughman]

For a complete line the HS1 has been mentioned but how about the Welsh Highland?

 

[Penmorfa]

For a complete line the HS1 has been mentioned but how about the Welsh Highland?

All built on old alignments, although one stretch is on its third track gauge!

 

[GRALISTAIR]

The Windsor Link opened in 1988 was a decent bit of track and now very heavily used:

http://en.wikipedia.org/wiki/Windsor_Link

Yes you beat me to it- great piece of line and as you say, heavily used and very functional - makes journeys from the north and northwest to Manchester Airport etc, very easy.

 

[507 001]

For a complete line the HS1 has been mentioned but how about the Welsh Highland?

All built on old alignments, although one stretch is on its third track gauge!

Actually, there are two parts of the welsh highland on new alignments.
First is croesor junction, the new railway is on a much wider curve than the old to allow a higher line-speed.
Second are the tramway sections which are all on a completely new alignment. The original alignment was entirely street running, and for the most part in the middle of the road iirc!

The latter only really opened in march last year

 

[dalmahoyhill]

airdrie bathgate opened two years ago. Most of it is on existing track but there are a couple of realignments